Inorganic scintillation crystal detectors, such as thallium-doped sodium iodide (NaI:Tl) scintillators have been widely used in a number of applications ranging from medical, geological (such as well drilling), waste management, radiation detection in foodstuff including meat, fruit, vegetables, and homeland security. Nuclear medicine involves the detection of photons, such as gamma rays, emitted from a source, such as an internal organ of a patient which contains a dose of a radiopharmaceutical compound. A scintillation camera uses a sodium iodide scintillator as a detector to absorb incident gamma ray photons from the object under study and interacts with the absorbed gamma rays to produce light events. The scintillator converts the energy of the gamma photon into a flash of light detected by a photo-multiplier tube array which views the side of the scintillating crystal away from the patient. In deep well drilling for resources such as petroleum drilling, a large quantity of information relating to geological formations and conditions of the drilled well is required for analyses. NaI:Tl scintillators as a fast neutron source can be scattered and absorbed in the well bore environment producing gamma rays, thus obtaining the physical characteristics of the well bore environment.
Scintillation counting employing NaI:Tl crystals has also been used to quantify contaminants in waste sites containing radionuclide contamination in a rapid and efficient manner so as to establish remedial protocols. Gamma-spectrometers using NaI:Tl scintillators have been used to test terrestrial vegetation, soil, milk, grain, vegetables, game animal sampling, and even road kills such as deer to detect the presence of radionuclides. To counter threat to homeland security with the smuggling of radioactive materials including dirty bombs across borders or into public buildings and facilities, portable and/or hand-held radiation detectors have been increasingly put in use as part of a comprehensive security system in borders and countries for scanning packages, containers, automotives, shipments, etc. These devices record collisions between gamma rays or neutrons and scintillation detectors such as sodium iodide crystals, producing flashes of light picked up by a photomultiplier and registered by a counter.
Sodium iodide scintillators used in these applications include thallium activated sodium iodide crystals, a technology dating back to 1948. In the prior art technology, NaI crystals of large sizes are grown in either the Bridgman-Stockbarger method, the Czochralski method, or other single-crystal techniques. Some of these technologies have their roots going back to the 1950's. However, all require the use of energy-intensive furnaces for growing the crystals, some with complex yield- and quality-critical control parameters such as temperature and thermal strain fields in the crystal, shape of growth interface, and convection patterns in the melt. There have been some publications disclosing methods for making sodium iodide crystals, among other alkali halide crystals in general. For example, U.S. Pat. No. 5,178,719, discloses a method for controlling residual impurity and dopant concentrations across the length of the crystal. However, most of the quality control solutions for making scintillator crystals have been kept as trade secrets.
Attempts were made in the 1950's to make scintillators from powders. UK Patent No. 792,071 discloses a process to make a transparent scintillation crystal by first compacting the activated powders under a pressure of ˜120,000 psi, then heat-treating the compact at a temperature up to the melting point of the crystal, e.g., from 200° C. to 650° C. for sodium iodide. JP Patent Publication No. S48-9272 discloses a process to apply pressure of “several tons per square cm” (1000 kg/cm2=14,223 psi) onto a body containing granular NaI crystals to prepare scintillator of a thickness of about 0.5 mm. Kashyuk et al. disclosed a process to compress alkaline chloride powder at 120° C. and 3000 to 12,000 kg/cm2 (42,670 to 170,680 psi) for 10 minutes to obtain transparent discs.
The prior art methods resulted in crystals of insufficient quality to be used in commercial applications. The invention herein provides a method to supply critically needed cubic halide scintillators of suitable quality.